The present invention relates to the use of heat pipes in the cooling of rotating elements such as rotors in electric motors. It should be emphasized that although the present discussion focuses on electric motors, the present technique affords benefits in heat removal in a number of systems employing rotating elements or components.
Electric motors of various types are commonly found in industrial, commercial and consumer settings. In industry, motors are employed to drive various kinds of machinery, such as pumps, conveyors, compressors, fans and so forth, to mention only a few. These motors generally include a stator having a multiplicity of coils surrounding a rotor. The rotor is typically supported by bearings for rotation in a motor frame. When power is applied to the motor, an electromagnetic relationship between the stator and the rotor causes the rotor to rotate. The speed of rotation of the rotor may be specified at predetermined speeds, for example, at 1200 revolutions per minute (rpm), 1800 rpm, 3600 rpm, and so on. On the other hand, the speed may be variable, such as where the motor is controlled via a variable frequency drive, for example. A rotor shaft extending through center of the rotor takes advantage of this produced rotation and translates the rotor's movement into a driving force for a given piece of machinery. That is, rotation of the rotor shaft drives the machine to which it is coupled.
During operation, conventional motors generate heat. By way of example, the physical interaction of the motor's various moving components produces heat by way of friction. Additionally, the electromagnetic relationships between the stator and the rotor produce currents that, in turn, generate heat due to resistive heating, for example. A particular source of resistive heating is the current flowing through the conductor bars disposed within the rotor. In general, excess heat left unabated may degrade the performance of the motor. Worse yet, excess heat may contribute to any number of malfunctions, which may lead to system downtime and require maintenance. Moreover, localized high operating temperatures (i.e., hotspots) sustained over time may lead to premature malfunction of the given location. Undeniably, reduced efficiency and malfunctions are undesirable events that may lead to increased costs.
To dissipate heat and to maintain the motor within acceptable operating temperatures, conventional motors route a coolant, such as forced air or liquid, through the stator and/or around the stator. Motor cooling designs have traditionally been directed toward the stator instead of the rotor because the stator is stationary and more accessible in operation, providing for relative ease in temperature monitoring and control. Moreover, motor applications have tended to be stator-limited with respect to operating temperature, and thus conventional approaches, such as shaft-mounted fan cooling, directed at the stator have generally been sufficient.
However, there are an increasing number of motor applications and designs where the rotor is prone to becoming excessively hot, and where cooling directed at the stator is not adequate in maintaining acceptable operating temperatures within the motor. Such rotor temperature-limited applications include, for example, high-power and/or high-density motors in the mining, heating, ventilating, and cooling industries. Other examples include hermetic motors and centrifuge motors. Hermetic motors are cooled by maintaining the motor submerged in a liquid or gas, and the inner part of the rotor (the part of the hermetically-sealed motor most removed from the refrigerant) tends to overheat. In the case of centrifuge motors, large starting times may result in undesirable temperature spikes within the rotor. Other motors that may experience high rotor temperatures include traction motors, Class I Division 2 motors having inverters, and so on. In general, motors in a variety of applications may experience excessive rotor temperatures and may benefit from direct cooling of the rotor. Placement of a heat pipe within the rotor to accomplish such direct cooling has generally been disregarded due to concerns about poor heat-pipe operation at the centrifugal forces associated with the relatively high rotating speeds of a rotor.
A heat pipe disposed in a rotating element may be classified as a rotating heat pipe or as a revolving heat pipe. A rotating heat pipe generally has the same center of rotation as the rotating element it is cooling. A revolving heat pipe generally does not. Operation of both types of heat pipes may be affected by centrifugal forces, especially at higher rotational speeds. The effects on heat-pipe operation, such as on the condensation, evaporation, and fluid flow of the internal liquid (e.g., water, ammonia, etc.), may be more pronounced with the revolving heat pipe because of its off-center radial position within the rotating element.
Some centrifugal force (e.g., less than one gravitational constant, g) may benefit operation of a revolving heat pipe. However, above 1 g, operation may become problematic. Thus, revolving heat pipes have generally not been employed to cool a rotor in an electric motor because, in part, a rotating rotor may generate centrifugal forces of up to 128 g and higher. In contrast, the rotating heat pipe, which would be typically positioned at the center of the rotor (e.g. in the rotor shaft), may exhibit more favorable heat transfer behavior at higher rotating speeds. However, a rotating heat pipe disposed at the center of the rotor would generally not provide adequate heat transfer area to sufficiently cool the rotor. Further, a rotating heat pipe disposed within the rotor shaft (e.g., a hollow rotor shaft configured as a rotating heat pipe) may compromise the structural integrity of the rotor shaft. In general, whether considering rotating or revolving heat pipes, heat pipe technologies have not made significant contribution to the thermal management of motors. Further, the thermal management of motors which are rotor-limited with regard to temperature remains unsatisfactory.
There is a need, therefore, for an improved technique for cooling an electric motor to accommodate excessive rotor temperatures. There is a need for direct cooling of the rotor within electric motors, such as through the use of heat pipes disposed in and/or around the rotor, providing for effective heat transfer at higher rotating speeds and greater centrifugal forces.